1. Field of the Invention
This invention relates to acoustic wave devices and more particularly to Group-III nitride based bulk acoustic devices and methods for fabricating the devices.
2. Description of the Related Art
Acoustic wave is a form of disturbance involving material deformation. Deformation occurs when motion of individual atoms are such that the distances between the atoms changes, accompanied by internal forces to restore the distance. If the deformation is time-variant, then these internal forces and effects determine motion of each atom. This gives rise to a wave motion with each atom oscillating about its equilibrium position. The material is then called “elastic” and the waves generated are called “elastic waves” or “acoustic waves”. There are two types of acoustic waves: longitudinal and shear waves based on the vibration and propagation direction of atoms and the wave respectively. One type is known as surface acoustic wave “SAW” that propagates along a plane surface of a homogeneous medium. Another type is referred to as bulk acoustic waves that propagate through a medium.
In electronic applications this phenomenon has been used to develop acoustic sensors/transducers, resonators, filters, etc. For frequencies below 50 MHz propagation medium can typically comprise quartz or glass. At higher frequencies crystalline media such as sapphire (Al2O3), zinc-oxide (ZnO) and lithium niobate are used. Such devices can operate at up to 5 GHz. These material systems make use of a phenomenon called “piezoelectricity”. Piezoelectricity in many materials couples elastic stresses and strains to electric field and displacements. It typically only occurs in anisotropic medium whose material structure lacks center of symmetry. Typical materials include ZnO, Lithium Niobate, and Group-III nitrides.
Surface acoustic wave (SAW) devices typically propagate onto the base of the piezoelectric material. SAW devices are used in high frequency components in modern communication systems due to their stability, reliability and compactness. SAW devices typically comprise a piezoelectric film on which two interdigitated transducers (IDT) are deposited so that high frequency AC electric fields can be converted into acoustic waves and vice versa. In a typical SAW device an input signal at the first IDT is transformed to an elastic perturbation that propagates along the surface of the piezoelectric film towards the output IDT. When voltage is applied at the input IDT an electric field is set up in each inter-electrode gap and surface waves are generated with amplitudes proportional to the voltage difference between the adjacent electrodes. These generated waves are received at the output IDT and an electrical signal is generated at the output IDT corresponding to the acoustic perturbation.
Recently there has been interest in developing Group-III nitride acoustic devices mainly due to superior electronic and acoustic properties. Group-III nitrides have piezoelectric properties and Group-III nitride acoustic devices have been developed, but mainly limited to SAW devices [Petroni et al., GaN Based Surface Acoustic Wave Filters for Wireless Communications, Superlattices and Microstructures, 36 (2004), 825-831; Palacios et al., Remote Collection and Measurement of Photogenerated Carriers Swept by Surface Acoustic Waves in GaN, Applied Physics Letters, Vol. 84, No. 16, 3162-3168]. These technologies, however, have several limitations including but not limited to limited power handling capability due to power density stored close to the propagation surface, increased sensitivity to surface contamination, and device dimensions (and frequency of operation) limited by current lithography techniques such as the space between IDTs. It is difficult for current lithography techniques space small enough between IDTs for higher frequency devices.
Bulk acoustic wave (BAW) devices are associated with waves traveling through a solid medium. Based on the choice of medium, attenuation can be low i.e. inherent propagation losses can be minimized. In addition, higher velocities can be achieved as well as higher frequencies when using thin film medium. Power handling can also be increased using BAW devices.
BAW devices have been developed using polycrystalline AlN as the active medium [Dubois, Thin Film Bulk Acoustic Wave Resonators: A Technology Overview, Memswave 03, Toulouse France, Jul. 2-4, 2003]. One significant disadvantage of this approach is the high propagation loss due to the polycrystalline nature of AlN films.